The present invention is directed to polyisocyanate addition compounds which i) are substantially free from isocyanate groups and are prepared from one or more monomeric polyisocyanates and/or polyisocyanate adducts, ii) contain allophanate groups, iii) contain siloxane groups (calculated as SiO, MW 44) in an amount of 0.002 to 50% by weight, and iv) contain ethylenically unsaturated groups (calculated as C═C, MW 24) in an amount of 2 to 40% by weight, wherein the preceding percentages are based on the solids content of the polyisocyanate addition compounds and wherein the siloxane groups are incorporated by reacting an isocyanate group with a compound containing one or more hydroxyl groups directly attached to a carbon atom and one or more siloxane groups to form urethane groups and/or allophanate groups, provided that more than 50 mole % of the groups that chemically incorporate siloxane groups into the polyisocyanate addition compounds are allophanate groups. The present invention also relates to the use of the polyisocyanate addition compounds in coating compositions curable by free radical polymerization.